This invention relates to a device for pressure measurement using a resistor strain gauge, more particularly of the type comprising a deformable substrate subjected to the pressure to be measured having a resistor strain gauge applied thereon, and an electric circuit capable of sensing the changes in resistance of the resistors in accordance with the substrate strain.
In such devices the strain caused by a pressure or load applied to the substrate results in a change in the size of the resistors and electrical properties thereof and accordingly a change in the resistance thereof.
Such a change is sensed by an electric or electronic circuit having the resistors connected thereto, so as to provide signals proportional to the substrate strain and accordingly to the pressure applied thereon.
The presently used devices for measurement of pressures or strains use, as strain gauges, metal wires, continuous metal films, discontinuous metal films, cermets and semiconductors.
The effect of the change in electric resistance of such elements as a result of a strain is commonly referred to as elastoresistance or piezoresistance.
Of course, an ideal strain gauge should have a substantial piezoresistive effect associated with a low thermoresistive effect. More particularly, it should for the first effect (strain sensitivity) have a high gauge factor ##EQU1## (wherein Ro and R are the resistance of the unstrained and strained resistors, respectively, and .epsilon.=.DELTA.1/1 is the relative elongation of the element) and for the second effect (thermal stability) low values both of the temperature coefficient of the resistor ##EQU2## (wherein .DELTA.R/R is the relative variation in resistance for a variation in temperature .DELTA.T) and of the temperature coefficient of Gauge factor ##EQU3## (wherein .DELTA.GF/GF is the relative variation of GF for a variation in temperature .DELTA.T).
Generally, the performances of the prior art strain gauges are highly dependent on the structure and composition of the resistors used.
Typical values of the most significant coefficients are shown in the following table, with reference to the conventional strain gauges of the aforementioned types.
TABLE ______________________________________ Long TCR TCGF term Resistors GF ppm/.degree.C. ppm/.degree.C. stability ______________________________________ Metal wires 2-5 20-4000 20-100 Optimum Continuous metal films 2-5 20-4000 20-100 Good Disc. metal films 100 1000 -- Very poor Cermet 100 1000 -- Poor Semiconductors 40-175 400-9000 200-5000 Good ______________________________________
The comparative analysis of the performances shows that discontinuous metal films and cermets cannot find wide fields of application due to the insufficient time stability of the electric and piezoresistive characteristics. Metal wires and continuous metals films are used where the strain sensitivity (GF) is not a critical requirement, but a good thermal behaviour is essential (low TCR and TCGF), while semiconductors are used for the high strain sensitivity thereof even though, due to the high value of TCR and TCGF, it is often necessary to resort to sophisticated and expensive temperature compensation techniques.
It is a further difficulty in the use of metal film and semiconductor strain gauges to find a good matching between the substrate and strain gauge. Thus, both should have the same thermal linear expansion, coefficient to avoid the arising of apparent strains (not connected with the occurrence of mechanical strains) due to the relative elongations caused by changes in temperature when the substrate and strain gauge are characterized by different coefficients of thermal expansion.